Carrier drift velocity balance mechanism in Si-based thin film solar cells using graded microcrystalline SiGe absorption layer

[Display omitted] •The graded i-SiGe absorption layer was used to balance the carrier drift velocity.•The improvement in balance mechanism was studied by the biased quantum efficiency.•The carrier recombination loss was reduced due to balanced-carrier drift velocity. The basic idea of balancing the carrier drift velocity in the absorption layer was proposed to improve the conversion efficiency of Si-based thin film solar cells. Using the graded microcrystalline i-SiGe absorption layer to modulate the energy band, the driven electric field of holes was increased from 5.92kV/cm to 7.26kV/cm, while the driven electric field of electrons was kept at 5.92kV/cm. Compared with the step i-SiGe absorption layer, the drift velocity ratio of electrons and holes was more balanced. The improvement mechanism of the p-Si/graded-i-SiGe/n-Si solar cells was further analyzed using the measurement of the biased quantum efficiency. Consequently, the short-circuit current density and the associated conversion efficiency of the p-Si/graded-i-SiGe/n-Si solar cells were improved from 21.40±0.47mA/cm2 to 26.36±0.56mA/cm2 and from 7.43±0.23% to 9.15±0.25%, respectively compared with the p-Si/step-i-SiGe/n-Si solar cells.